The present invention relates in contradistinction to superheterodyne receivers to a directly mixing receiving system in which a high frequency (HF) received signal is converted directly to the base band by multiplication with a synchronized local oscillator signal. The baseband is the frequency band seized by the information to be transmitted, for example the video band from DC to 6 MHz for television or the audioband from 20 Hz to 20 kHz at FM-broadcasting.
Systems of the type to which the invention is directed employ a phase locked loop (PLL) phase control circuit including a mixer connected in series with a lowpass filter for suppressing the portion of the mixed product at the sum frequency and a voltage controlled oscillator (VCO) whose signal frequency or phase can be tuned to or locked on the immediate vicinity of the frequency, or the precise frequency, or a fixed phase difference with respect to the phase of the received HF signal, or of its existing or imaginary carrier.
Such a receiving system is disclosed in the article "Phase Locked AM Radio Receiver" by L. P. Chu in IEEE Transactions on Broadcast and TV Receivers, Vol. 15, 1969, pages 300-308. This receiver employs a complicated Costas phase control loop including a first mixer in which a local oscillator signal in phase with the carrier of the received signal is mixed with the received signal, a first lowpass filter with series-connected low frequency (LF) amplifier, a second mixer in which the local oscillator signal delayed by 90.degree. is mixed with the received signal and which is connected in series with a second lowpass filter and a second LF amplifier, as well as a phase detector which compares the outputs of the two LF amplifiers and feeds the result to a control filter with series-connected varactor which controls the local oscillator. Thus, the Costas control loop includes two control loops, i.e. an "in-phase channel" and a "quadrature channel." The Costas loop is locked in the correct phase if a zero signal is produced in the quadrature channel. The demodulated LF signal can be obtained at the output of the in-phase channel. The circuit is designed to receive double sideband, amplitude modulated, received HF signals as defined by the mathematical derivation of the mode of operation of this synchronous demodulator at page 301, right-hand column. The above-mentioned article does not contain any mention of the usability of such a synchronous demodulator in a system for receiving signals modulated in another manner.